The API I spent the most time scratching my head over was fl.drawingLayer, which is used for rendering the temporary outlines that appear when (for example) you're dragging out a box with the Rectangle tool. It has some quirks.
fl.drawingLayer.beginFrameclears this array.
fl.drawingLayer.endFramedraws the contents of this array to the screen.
fl.drawingLayer.beginDraw, according to its documentation, "puts Flash in drawing mode", and sets the Boolean
persistentDrawoption, which will cause flash to leave your drawing on the stage after the mouse button is released. In practice it seems to make no difference.